The present invention relates to a measuring apparatus, and more particularly to an apparatus for measuring the length of a sheet.
Various kinds of measuring apparatus for measuring the size or the squareness of a rectangular or square sheet are known. One such measuring apparatus is known from for example, Japanese Pat. Publ. No. 60-57003. The measuring apparatus described in this publication consists of four optical angle sensors for detecting corner angles of the sheet and an arithmetic circuit for calculating not only the differences between actual lengths and reference lengths for the four sides of the sheet based on the detected corner angles but also the squareness of the sheet. To place the sheet accurately on a measuring table, two guide rollers are disposed along two sides of the measuring table perpendicular to each other.
Another such measuring apparatus is known from Japanese Unexam. Pat. Publ. No. 58-191,908. The measuring apparatus described in this latter publication consists of two cameras having photodiode arrays to detect front and rear sides of a sheet in synchronism with each other. To improve the accuracy of measurement, it is adapted to collect or pick up data from the cameras at a time interval such that the difference between times at which the cameras detect the front and rear sides of the sheet, respectively, will be the least.
The sheet is usually cut from a long web of sheet material by means of a shearing or cutting machine. One such cutting machine is known from Japanese Unexam. Pat. Publ. No. 60-155,314. The cutting machine described in that publication includes scanning cameras which serve as web position correcting means located upstream of the position wherein the long web is cut to sheets, for measuring both sides of a long web running at a constant speed to position the web accurately in the cutting position.
There are, however, various disadvantages or problems in connection with the above-described apparatus. That is, the measuring apparatus described in Japanese Pat. Publ. No. 60-57003 positions the sheet on the measuring table when the sheet conveying system is stopped. The apparatus is therefore inconvenient to use in measuring sheets separately one by one. The measuring apparatus described in Japanese Unexam. Pat. Publ. No. 58-191908, on the other hand has difficulty in accurately measuring when a sheet runs at an inclination to the direction of lengthwise movement. Even in the Japanese Unexam. Pat. Publ. No. 60-155,314, accurate measurement is difficult because of variations in sheet lengths.